[1] Field of the invention
The present invention relates to a camera having a liquid crystal focusing screen.
[2] Description of the Prior Art
FIG. 1 is a sectional view of an optical system of a single-lens reflex camera.
A light bundle passes through a photographing lens 10 and is divided into two parts on a main mirror 11. One is reflected by the main mirror 11 to the viewfinder side, and then, forms an image on a focusing screen 12. The formed image is observed through a pentaprism 13 and an eyepiece 14. Close to the eyepiece 14, a lens for light measuring 15 and a light measuring sensor 16 are provided. The lens for light measuring 15 and the light measuring sensor 16 measure the light bundle led from the pentaprism 13. The other light bundle which was divided passes through the translucent main mirror 11, and is reflected by a sub mirror 17, and then, is led to an AF module 18 which carries out focus detection. The photographing lens 10 represents lens groups consisting of plural lenses. An aperture 19 is usually provided among the plural lenses of the photographing lens 10. A film provided at the optical axis of the photographing lens 10 is not shown in FIG. 1.
The focusing screen 12 is a plastic plate and has a subtly uneven surface thereon so as to cause light diffusion. When a photographer observes an object through a viewfinder, an aperture of the camera is usually opened fully. This is for keeping the view finder bright and for achieving higher precision by strengthening output from the light measuring device (the light measuring sensor 16 in FIG. 1) in light measuring. However, when a shutter is released, the aperture is stopped down to the most suitable size in exposure control or the size selected by the photographer. Therefore, the photographer can not confirm the depth of field of a printed picture before the shutter is released. A method of stopping down the aperture with a preview button (not shown) to the practically controlled size in photographing so as to confirm the depth of field through the viewfinder is known. By this method, the viewfinder turns dark and it is hard to observe an object therethrough. Another problem is that the depth of field observed through the viewfinder varies when the open aperture value is altered according to a change of photographing lenses.
To confirm the depth of field of a printed picture through the viewfinder without decreasing the brightness of the viewfinder and the output of the light measuring device, a method of varying a light diffusion degree of a liquid crystal focusing screen has been proposed.
In case of using a liquid crystal member as a focusing screen, the following four points should be considered.